You should be able to use use the 2 RED/WHITE RCA jacks on the back of the BR player to get stereo sound. (I've never seen a device that didn't have these) Then you'll want a cable to convert your pc from 3.5mm headphone to RCA. Use the RED/White from the xbox 360 and plug all of those into one of these:
Use this cable to go from the pc to the switchbox:
To go from the switchbox to the speakers you'll need that same cable but with the female 3.5mm end.
The whole project should cost about $10 depending upon the cable length needs.